Studies proposed in Project 3 will investigate the fundamental question that Ca2+ entry mediated by transient receptor potential cation channel 6 (TRPC6), a receptor-operated channel (ROC), is a critical determinant of increased endothelial permeability. We will test the hypothesis that TRPC6 is the critical influx pathway for Ca2+ that triggers the activation of endothelial contractile mechanisms RhoA and endothelial isoform of MLCK (EC-MLCK). If this hypothesis is correct, it would demonstrate a fundamental link between TRPC6 and the RhoA-EC-MLCK pathway. We also postulate based on our Supporting Data a potentially important relationship by which TRPC6-activated signaling upregulates the activity of TRPC1, a store-operated channel (SOC). Thus, our concept is that TRPC6 by activating both RhoA and EC-MLCK controls the state of endothelial contraction and additionally that TRPC6 activates signals that upregulate TRPC1 activity, thereby protracting and/or amplifying the endothelial permeability response. Our Specific Aims are to determine: 1) the role of the endothelial ROC, TRPC6, interaction with the SOC, TRPC1, in mediating Ca2+ signaling-dependent increase in lung vascular permeability; 2) the role of TRPC6-activated signaling in mediafing RhoA and ECMLCK activation and the requirement of these effectors in mediating increased lung vascular endothelial permeability and 3) the role of EC-MLCK downstream of TRPC6 in upregulating TRPC1 activity, and its consequences in increasing lung endothelial permeability. These studies will be carried out utilizing endothelial cells and lungs from mice carrying deletions of specific genes. We believe that the studies will provide a new understanding of the Ca2+-mediated increase in lung vascular permeability and edema formation. These new findings will help to unravel the details of how the interactions between TRPC6 and TRPC1 signals the Ca2+-dependent increase in lung endothelial permeability. The studies are expected to offer a novel therapeutic strategy for inhibiting TRPC6 and TRPC1 activities, thereby abrogating or reversing lung vascular leakiness.